The present invention relates to a stationary induction apparatus, such as a high-voltage transformer, which should be contained in a housing.
A stationary induction apparatus has windings which are made of electric wires covered with an insulating coating and which are immersed in insulating fluid (insulating medium) such as oil. The windings are heated with an electric current and cooled with the insulating fluid. The insulating fluid flows by convection in the apparatus, is cooled in a heat-exchanging cooler provided in the housing of the stationary induction apparatus or outside the stationary induction apparatus, and flows back to the windings. The insulating material is an organic material in most cases and gradually decomposes. The rate at which the insulating material decomposes is greatly influenced by the amount of oxygen mixed into the insulating material and the temperature of the insulating material. If the apparatus malfunctions, a part of the insulating material may undergo dielectric breakdown. In this case, the part of the insulating material is damaged or decomposes. When the material is damaged or decomposes, a high-frequency signal is generated in the electric line, and an electromagnetic wave emanates from the discharging part. The electromagnetic wave decreases in magnitude in inverse proportion to the square of the distance from the position where it has generated. Hence, a small breakdown can be detected if the electromagnetic wave is caught near the position where it has generated.
On the conventional stationary induction apparatus, a monitoring device is mounted outside the stationary induction apparatus to monitor the operating state of the stationary induction apparatus. The monitoring device monitors the apparatus, either periodically or continuously. The monitoring device analyzes the temperature and composition of the insulating fluid, thus evaluating the functional efficiencies of the fluid, such as insulating efficiency. The insulating material is liable to degradation or damages because a high voltage is applied and current flows through the windings. As the material is degraded or damaged, it decomposes, generating some substances. These substances flows into the insulating fluid and are carried, by convection of the fluid, to a sensor unit that is provided in the housing.
The heat generated from the current is also transferred by the convection in the insulating fluid. The insulating fluid does not flow at a uniform speed in the housing. That is, the insulating fluid flows at different speeds at different positions in the housing. The insulating fluid starts undergoing convection when the fluid receives the heat generated at the lower parts of the windings. Since heat is generated at the entire windings, not at parts thereof, the insulating fluid receives heat not only by heat convention but also directly from the windings. The fluid reaches the highest temperature where the fluid flows to the upper parts of the windings. The heat is transmitted not only by convection in the fluid, but also by radiation from the fluid. Therefore, in most cases, the top center part of the apparatus is most heated than any other part of the apparatus. Any organic insulating material that is used in ordinary stationary induction apparatuses decomposes with time. The rate with which the material decomposes depends on its temperature.
The insulating material is likely to decompose and an abnormal signal is likely to develop at high-voltage regions of the windings. The monitoring device needs to be located at a ground-potential position. The device should be electrically insulated from a sensor. Most sensors include a detecting unit, a converting/amplifying unit, and a signal-transmitting unit. These sensor parts require energy to operate. They acquire energy from the monitoring device, from the environment in which they are used, or from the signals that they have detected. In the environment, the sensor parts obtain energy from vibration, temperature difference, light or electromagnetic wave emanating from an electric or magnetic field. If the sensor parts acquire energy from a control panel, they receive the energy directly from a power supply or the energy converted in the sensor unit from light. Signals may be transmitted as electric signals, optical signals or mechanical-displacement signals. Optical signals are transmitted as modulated luminance signals or modulated color signals.
Generally, sensors using electric signals cannot be disposed at or near any high-voltage regions. A sensor that is disposed at a high-voltage region, for example, has an insulating member such as optical fiber. However, a sensor of this type is not fit for monitoring general-purpose apparatuses, because the sensor of this type needs to transmit optical signals, the optical fiber also must be insulated, and signals are acquired in only a few types. Further, the sensor of this type cannot directly receive electric energy. The sensor of this type receives optical energy or uses, as energy source, the electric field available at the position where the sensor is provided.
Typical stationary induction apparatuses have magnetic shields at various positions, for inducing leakage magnetic fluxes. The windings have electrostatic shields for moderating electric fields. Therefore, communication using an electromagnetic technique cannot be achieved in the regions that these shields protect.
As can be seen from the above, it is extremely difficult to provide sensors capable of detecting various signals and condition to monitor the operating state of the stationary induction apparatus, at high-voltage regions. Typical stationary induction apparatuses are used for a long time and are so huge that no access is easy to their interior. Therefore, signals can hardly be electrically transmitted from sensors so that the sensors may be disposed at high-voltage regions. To transmit the signals, the signals must be converted to such signals as can be easily transmitted while maintaining their insulation state. This signal conversion requires energy. The sensor unit therefore needs to incorporate a battery. Alternatively, power needs to be generated at the sensor unit.
In order to transmit signals optically, the optical fiber used needs to be insulated well. To transmit signals mechanically, the mechanical component working as transmission medium needs to be made of insulating material. Such a mechanical component can hardly be provided easily at low cost. Electromagnetic signals may be transmitted. However, they are likely to contain noise because the high-voltage components are connected to external electric lines. Moreover, signals cannot be so easily transmitted from the sensors, because the apparatus generates low-frequency signals and an intense low-frequency magnetic field, has a structure for shielding the signals and the magnetic field, and includes electrical conductors that hardly allow electromagnetic waves to leak, making it difficult to transmit signals outside.
If a sensor is disposed at a high-voltage region, drive energy needs to be supplied to the sensor unit. Electrical connection for supplying the energy to the sensor unit cannot be accomplished, because the sensor unit is set at high voltage. The energy can be supplied in the form of light if a solar cell is disposed at the sensor unit and the sunlight is applied to the solar cell via an optical fiber. The solar cell can hardly be used in practice, because the solar cell has but low energy-conversion efficiency and generates heat. Power may be generated from the electric or magnetic field of the commercial frequency the apparatus transforms. This method of generating power can hardly be put to practical use, because the operating current changes at all the times and the unit for generating power is required to be manufactured at high precision.
In consideration of the foregoing, IC (integrated circuit) tags and IC tag systems have been invented, as publicly known. The IC tag has a sensor, is set in an apparatus to acquire information about the interior of the apparatus, uses electromagnetic waves generated in the apparatus as its operating energy, and records the magnitude of each electromagnetic wave and the number of electromagnetic waves. The drive energy for the IC tag having a sensor is the power supplied from a battery or a communication electromagnetic wave.
The IC tags having a sensor, disclosed in Japanese Patent Application Laid-Open Publication Nos. 2004-133596 and 2002-130675, the entire contents of which are incorporated herein by reference, are based on the assumption that the tag only transmits detected information and that the user at a remote site determines whether a trouble has developed in an apparatus. Hence, the IC tag has no functions of automatically recording or accumulating the information. If the IC tag has a memory function, the tag keeps holding the operating history of the apparatus even if after the tag has been removed from the apparatus.
Japanese Patent Application Laid-Open Publication No. 2006-185048, the entire content of which is incorporated herein by reference, discloses a technique of using the electromagnetic waves generated in the apparatus as operating energy. Further, the electromagnetic waves generated as information representing a trouble in the sensor that records the magnitude of each electromagnetic wave and the number of electromagnetic waves is used as operating energy, too. Therefore, no operation is performed if no troubles develop.
The above-identified Japanese Patent Application Laid-Open Publications propose that a sensor should be disposed at a region where any sensor has hitherto been hardly provided. However, any electromagnetic environment that may influence the communication or the sensor is not taken into consideration. Particularly, no technique is disclosed, which may prevent an intense electric field, other than one used for communication and transmitting drive energy, from disabling the communication with the apparatus or from damaging the electronic circuit provided in the apparatus.
If an IC tag is incorporated in a stationary induction apparatus, the IC tag should be set at a position electromagnetically shielded or a position where no electromagnetic waves reach, or should be protected from electromagnetic waves by taking some measures. Unless the IC tag is so positioned or so protected, the IC tag may be broken by the intense electric field or intense magnetic field that the main apparatus generates.